1. Field of the Invention
The present invention relates to a method for the manufacture of thermoplastic resin sheets with embossed surfaces. More particularly, it relates to a method for the manufacture of the type of thermoplastic resin sheets suitable for use in the laminated glass products which are widely applied in the automotive, aircraft, construction and other industries.
2. Description of the Prior Art
Laminated glass is fabricated by interposing a thermoplastic resin sheet as an intermediate layer between two plates of glass, and then applying pressure thereto. In the manufacture of laminated glass, the air between the glass plates and the resin sheet must be adequately expelled, and therefore thermoplastic resin sheets with embossed surfaces are employed.
The following methods have been proposed for the manufacture of thermoplastic resin sheets with embossed surfaces.
(1) First, a thermoplastic resin sheet with a flat surface is manufactured by means of an extruder or mold. Then, an embossed pattern is impressed upon the surface of this sheet by passage between a pair of embossing rollers (Japanese Laid-Open Patent Publication No. 61-61835).
(2) A resin composition containing 0.6-15% by weight of water is extruded from an extruder, and compressed in a mold, resulting in an embossed sheet (U.S. Pat. No. 2,904,844).
(3) The lips of an extrusion mold are maintained at a temperature lower than that of the resin being extruded from these lips, thereby creating a temperature difference between the inside and outside of the resin. A desired sheet is obtained by extruding molten resin from the lips under these conditions (Japanese Laid-Open Patent Publication No. 61-154919).
In the aforesaid method (1), if the thermoplastic resin possesses high tack, then, when passing between the embossing rollers, the sheet may adhere and wind onto the rollers, thus causing malfunctions in the manufacturing process. Since it is difficult for the sheet to be detached from the embossing rollers, blemishes are formed and the appearance of the sheet deteriorates. Moreover, when wide sheets are manufactured by this process, large rollers of width exceeding that of the sheets are required, which entails high equipment expenditures and running costs.
In the aforesaid method (2), since the resin contains water, this water may cause foaming of the sheet. If dehydration adequate to prevent the foaming of the sheet is performed within the extruder, then the embossment of the sheet so obtained is not sufficiently rough.
As for the aforesaid method (3), the lip of the extrusion mold must be cooled sufficiently to ensure that the temperature difference between the resin and the lip is approximately 40.degree. C. Therefore, cooling ducts must be incorporated into the mold, which makes the structure of the mold complex, and moreover, temperature regulation devices must also be provided in the mold, which makes equipment costs for this process high.
When the lips are finished by lapping or polishing, if the edges of the lips on the side of the resin extrusion orifice are unduly sharp, then chipping or burrs are prone to occur, and therefore, as shown in FIGS. 8a and 8b, the edges of the lips 13 are generally rounded 14 or bevelled 15. However, if sheets are fabricated by extrusion of a tacky thermoplastic resin (e.g., polyvinyl butyral or ethylene vinyl acetate copolymers) from a mold 16 such that the lips 13 have been rounded 14 or bevelled 15, then, as indicated in FIG. 9a, scraps of resin (lip debris) 17 adhere to the edges of the lips of the mold 16. If these scraps 17 accumulate on the edges of the lips 13, as shown in FIG. 9b, this develops into so-called lip residue 18, and this residue 18 adheres to the sheet 10 extruded from the slit, resulting in the production of defective sheets 10. Therefore, the lips 13 of the mold 16 must be adequately cleaned to avoid adhesion of lip debris 17 to the sheets 10, which entails a substantial drop in productivity.
The following methods have been proposed in order to prevent adhesion of resin scraps to the lips.
(4) An anti-adhesive solution composed of fluorocarbon resin or other suitable substance is applied to the surfaces of the lips (Japanese Laid-Open Patent Publication No. 58-12743).
(5) Anti-adhesive parts composed of tetrafluoroethylene resin are attached to the edges of the lips (Japanese Laid-Open Patent Publication No. 56-95637).
In the aforesaid method (4), since a special process is required for the application of an anti-adhesive solution to the lip surfaces, the cost of the molds is increased. Likewise, in the aforesaid method (5), the attachment of anti-adhesive parts to the mold requires special work, thereby raising the cost of the mold, and moreover, the cleaning of the lips becomes difficult.